Passive waveguide structure with alternating gainas/alinas layers for mid-infrared optoelectronic devices

ABSTRACT

Disclosed is a semiconductor optical emitter having an optical mode and a gain section, the emitter comprising a low loss waveguide structure made of two alternating layers of semiconductor materials A and B, having refractive indexes of Na and Nb, respectively, with an effective index N o  of the optical mode in the low loss waveguide between Na and Nb, wherein No is within a 5% error margin of identical to a refractive index of the gain section and wherein the gain section is butt-jointed with the low loss waveguide, and wherein the size and shape of the optical mode(s) in the low loss waveguide and gain section are within a 10% error margin of equal. Desirably, at least one of the semiconductor materials A and B has a sufficiently large band gap that the passive waveguide structure blocks current under a voltage bias of 15 V.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/628,394, filed on Feb. 23, 2015, which claims the benefit of U.S.Provisional Patent Application No. 61/946,700 filed on Feb. 28, 2014,the contents of each being incorporated herein by reference.

This application is also related to Provisional Application Ser. No.61/732,289 filed on Nov. 30, 2012, and to Application No.PCT/US2013/072195 filed on Nov. 27, 2013, the contents of each beingincorporated herein by reference.

FIELD OF THE INVENTION

The present specification generally relates to quantum cascade lasers(“QCLs”) and particularly to passive waveguide structures for use inQCLs and to QCLs using such structures.

TECHNICAL BACKGROUND

A quantum cascade laser is a unipolar device. It utilizes intersubbandtransitions, unlike the traditional direct band gap semiconductorlasers, and it usually emits in the mid-infrared (“mid-IR”) orfar-infrared (“far-IR”) wavelength range.

Mid-IR sources are of interest for various reasons. Strong absorptionlines in the mid-IR region from the vibration of chemical bonds can beused to identify molecular composition. For example, FIG. 1 (prior art)shows a strong absorption line of CO₂ near 4.3 um. A single wavelengthMid-IR light source such as a QCL can be used to detect gas moleculessuch as CO₂ by detecting the absorption of a characteristic wavelengthsuch as 4.3 um.

To achieve single wavelength emission, grating structures may be addedto the QCL in the active region to make a distributed-feedback (“DFB”)quantum-cascade laser (“DFB QCL”). DFB QCLs generally emit a singlewavelength and can only be tuned over a small wavelength range, whichallows them to be used to detect a single species of small gas moleculesuch as CO₂. However, some big molecules in solid or liquid phases havewide and/or complex absorption spectra, like the explosive substances inFIG. 2, for example, which shows infrared absorption spectra for PETN102, RDX 104, TATP 106 and TNT 108. For detecting and differentiatingsubstances with such wide and/or complex absorption spectra, QCLs withboth single wavelength emission and wide tuning range are desirable.Range R indicated in the figure, for example, may be used to detect, anddifferentiate among, the spectra shown.

External cavity QCLs can have both single wavelength emission and widetuning range, but they are usually expensive and bulky. A distributedBragg reflector (“DBR”) QCL has one or both reflective gratings outsidethe gain region of the laser, allowing for a more independent thermaltuning of the gratings and a wider tuning range than a DFB QCL. A DBRQCL is thus a potential alternative to external cavity QCLs with theadvantages of relatively low cost and a compact, robust, monolithicform.

DBR QCLs typically have an essentially uniform, common core, as shown inFIG. 3 (prior art). The grating layers on DBR sections are formeddirectly on the layer(s) of the common core. Since the region of commoncore under the DBR is passive in operation (not part of the gain region)receiving no or minimal pump current during operation (due to additionalassociated current blocking structures or the like), it has relativelystrong resonant absorption.

Implementing a waveguide different from the waveguide of the activeregion in a DBR QCLs is disclosed in the related applications referencedabove. By using for the DBRs a different waveguide transparent (or atleast more transparent than the active region waveguide) to wavelengthsin the operating wavelength range, absorption losses in the DBRs can bereduced, allowing higher maximum power and wider total tuning (lasing)range in the laser device.

SUMMARY

To provide the benefits described above of including in a DBR QCL atransparent waveguide as well as to provide for similar benefits of atransparent waveguide in other semiconductor active optical devices, thepresent disclosure includes a transparent (or relatively transparent)waveguide structure made of two alternating layers of semiconductormaterials A and B, which have refractive indexes of N_(a) and N_(b).Desirably, at least one of A and B should have a relatively large bandgap so that the passive waveguide structure can block electric currentvery well, even under relatively high voltage bias. The effective indexof the optical mode in the passive waveguide N_(o) will be in-betweenN_(a) and N_(b); for good propagation of the optical mode, N_(o) shouldbe equal (or close) to the refractive index in an associated gainsection which is butt-jointed with the passive waveguide. Also for goodpropagation of the optical mode, the size of the optical mode(s) in thepassive and gain section should be equal or reasonably so.

An embodiment of the invention provides a semiconductor optical emitterhaving an optical mode and a gain section, the emitter including a lowloss waveguide structure made of two alternating layers of semiconductormaterials A and B, having refractive indexes of N_(a) and N_(b),respectively, with an effective index N_(o) of the optical mode in thelow loss waveguide between N_(a) and N_(b), wherein N_(o) is within a 5%error margin of identical to a refractive index of the gain section andwherein the gain section is butt-jointed with the low loss waveguide,and wherein the size and shape of the optical mode(s) in the low losswaveguide and gain section are within a 10% error margin of equal.

These and other features and advantages will be apparent from thespecification and the drawings to those of skill in the art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 (Prior Art) is a graph of absorption spectrum for CO₂ in theinfrared.

FIG. 2 (Prior Art) is a graph of absorption spectra of various explosivecompositions in the infrared.

FIG. 3 (Prior Art) is a cross-sectional schematic diagram of a DBR QCL.

FIGS. 4A, 4B and 4C are schematic cross-sectional views of variousalternative aspects of certain embodiments of a device according to thepresent disclosure.

FIGS. 5A and 5B are graphs of the optical mode profiles of someembodiments of structures according to the present disclosure, asgenerated by computer simulation.

FIG. 6 is a graph of a pulsed V/I curve test of an embodiment of apassive waveguide structure according to the present disclosure.

FIG. 7 is a graphed curve of a pulsed LIV test for a DBR QCL with apassive waveguide structure according to the present disclosure and acomparative DBR QCL.

DETAILED DESCRIPTION

The present invention can be understood more readily by reference to thefollowing detailed description, drawings, examples, and claims, andtheir previous and following description. However, before the presentcompositions, articles, devices, and methods are disclosed anddescribed, it is to be understood that this invention is not limited tothe specific compositions, articles, devices, and methods disclosedunless otherwise specified, as such can, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular aspects only and is not intended to be limiting.

The following description of the invention is provided as an enablingteaching of the invention in its currently known embodiments. To thisend, those skilled in the relevant art will recognize and appreciatethat many changes can be made to the various aspects of the inventiondescribed herein, while still obtaining the beneficial results of thepresent invention. It will also be apparent that some of the desiredbenefits of the present invention can be obtained by selecting some ofthe features of the present invention without utilizing other features.Accordingly, those who work in the art will recognize that manymodifications and adaptations to the present invention are possible andcan even be desirable in certain circumstances and are a part of thepresent invention. Thus, the following description is provided asillustrative of the principles of the present invention and not inlimitation thereof.

Disclosed are materials, compounds, compositions, and components thatcan be used for, can be used in conjunction with, can be used inpreparation for, or are embodiments of the disclosed method andcompositions. These and other materials are disclosed herein, and it isunderstood that when combinations, subsets, interactions, groups, etc.of these materials are disclosed that while specific reference of eachvarious individual and collective combinations and permutation of thesecompounds may not be explicitly disclosed, each is specificallycontemplated and described herein. Thus, if a class of substituents A,B, and C are disclosed as well as a class of substituents D, E, and F,and an example of a combination embodiment, A-D is disclosed, then eachis individually and collectively contemplated. Thus, in this example,each of the combinations A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F arespecifically contemplated and should be considered disclosed fromdisclosure of A, B, and/or C; D, E, and/or F; and the examplecombination A-D. Likewise, any subset or combination of these is alsospecifically contemplated and disclosed. Thus, for example, thesub-group of A-E, B-F, and C-E are specifically contemplated and shouldbe considered disclosed from disclosure of A, B, and/or C; D, E, and/orF; and the example combination A-D. This concept applies to all aspectsof this disclosure including, but not limited to any components of thecompositions and steps in methods of making and using the disclosedcompositions. Thus, if there are a variety of additional steps that canbe performed it is understood that each of these additional steps can beperformed with any specific embodiment or combination of embodiments ofthe disclosed methods, and that each such combination is specificallycontemplated and should be considered disclosed.

In this specification and in the claims which follow, reference will bemade to a number of terms which shall be defined to have the followingmeanings:

“Include,” “includes,” or like terms means encompassing but not limitedto, that is, inclusive and not exclusive.

The term “about” references all terms in the range unless otherwisestated. For example, about 1, 2, or 3 is equivalent to about 1, about 2,or about 3, and further comprises from about 1-3, from about 1-2, andfrom about 2-3. Specific and preferred values disclosed forcompositions, components, ingredients, additives, and like aspects, andranges thereof, are for illustration only; they do not exclude otherdefined values or other values within defined ranges. The compositionsand methods of the disclosure include those having any value or anycombination of the values, specific values, more specific values, andpreferred values described herein.

The indefinite article “a” or “an” and its corresponding definitearticle “the” as used herein means at least one, or one or more, unlessspecified otherwise.

The present disclosure includes a transparent waveguide structure (orrelatively transparent, relative to a non-energized active or gainsection waveguide) made of two alternating layers of semiconductormaterials A and B, which have refractive indexes of N_(a) and N_(b).Desirably, at least one of A and B should have a relatively large bandgap so that the passive waveguide structure can block electric currentvery well, even under relatively high voltage bias. The effective indexof the optical mode in the passive waveguide N_(o) will be in-betweenN_(a) and N_(b); for good propagation of the optical mode, N_(o) shouldbe equal (or close) to the refractive index in an associated gainsection which is butt-jointed with the passive waveguide. Also for goodpropagation of the optical mode, the size of the optical mode(s) in thepassive and gain section should be equal or reasonably so.

In the case of Mid-IR-emitting QCLs grown on InP substrates, thetransparent waveguide core should be (on average) lattice-matched toInP, using compounds, such as AlGaInAs or GaInAsP or AlGaIn(P)Sb, withthe composition(s) being adjusted for the desired refractive index(matching the corresponding active or non-transparent waveguide) and forlattice-matching to InP. For a rather short wavelength emitting QCL core(λ=4.5 μm), GaInAsP or AlGaInAs with a room temperature bandgap of about0.95-1 eV (corresponding to a photoluminescence wavelength around 1.28μm) has the appropriate refractive index, but for a QCL core emittingaround λ=10-11 μm, the bandgap of the appropriate GaInAsP or AlGaInAsmaterial should be around 0.8-0.9 eV (corresponding to aphotoluminescence wavelength around 1.45 μm).

Regarding providing the desirable insulating or semi-insulting nature ofthe transparent waveguide, InP and AlInAs can be grown semi-insulating.Although AlInAs has been grown semi-insulating at low growthtemperature, whether taking advantage of native defects or of Ccontamination, AlInAs is usually grown semi-insulating by the additionof dopant atoms such as Fe, Ti, Ru or other transition metals, whichcreate deep traps which trap free carriers; this is also the case forInP. It has been shown (see, for examples, [B.Tell, U. Koren and B.I.Miller, Metalorganic vapor-phase-epitaxial growth of Fe-dopedIn0.53Ga0.47As, J. Appl. Phys 61, 1172, 1987], [D.G. Knight, W.T. Mooreand R.A. Bruce, Growth of semi-insulating InGaAsP alloys using lowpressure MOCVD, J. Crystal Growth 124, 352, 1992]) that GaInAsP with lowbandgap (<0.8-0.9 eV) is difficult to dope for semi-insulatingqualities; if semi-insulating at room temperature, it becomes conductingat higher temperature (400 K), which is the temperature at which a QCLcore is likely to operate. However, the indices of InP and AlInAs aretoo low to match the index of the laser's active core. So they cannotact as the core material of a passive waveguide.

The solution to this particular problem provided by the presentdisclosure is to use as a transparent waveguide not a homogeneousmaterial but a stack of AlInAs and GaInAs layers. The GaInAs is leftundoped while the AlInAs is desirably doped with a deep trap element.Undoped AlInAs as grown in an reactor is lightly n-type, and it isaccordingly chosen as dopant Fe, which acts as trap for electrons. Ifthe thickness of semi-insulating material is high enough (0.5 μm forexample), tunneling through it does not occur and the resulting stack isinsulating enough. As alternative embodiments, different stacks could bechosen, such as GaInAs/InP; AlGaInAs/AlGaInAs or GaInAsP/GaInAsP—ofdifferent compositions, low bandgap/high bandgap—or another combination.

A transparent or low loss passive waveguide structure with a core madeof alternating undoped (or doped for semi-insulating behavior)GaInAs/AlInAs layers, is shown in FIG. 4C. The core is sandwichedbetween top and bottom InP cladding layers (n⁻ doped) (note that the nwith the superscript “−” is generally understood in the art as low ntype doping), like a QCL active core. The ratio of the thicknesses ofGaInAs and AlInAs is designed in such a way that the effective index ofthe optical mode in the passive waveguide equals that in the waveguidewith QCL laser core. When AlInAs is appropriately doped forsemi-insulating behavior, the core of the passive waveguide can blockelectric current up to certain voltage bias (>20 V), so that no leakagecurrent can go through the passive waveguide. Therefore no additionalcurrent blocking (isolation) structures are needed, and the fabricationof the devices can be simplified.

As seen in FIG. 4A, the low loss waveguide may be used for the front andrear DBR gratings. As seen in the alternative of FIG. 4B, the low losswaveguide may also be used for the phase section, if desired (and if thephase section is controlled via a microheater rather than currentinjection, in the case the low loss waveguide is insulating (that is,semi-insulating).

The passive waveguide will have low optical loss according to thepresent disclosure, which is mostly due to reduced free carrierabsorption. Since the GaInAs/AlInAs material is either undoped or isdoped to produce semi-insulating characteristics, the optical loss inthe low-loss waveguide core is negligible. The effective refractiveindex of the passive waveguide can be adjusted between 3.1 (therefractive index of AlInAs) and 3.3 (the refractive index of GaInAs), bychanging the ratio of the thicknesses of AlInAs and GaInAs. Thereforethe effective index of the passive waveguide can be easily designed tomatch the effective index of the optical mode in the active(light-emitting) waveguide (core). The passive waveguide according tothis embodiment, when the AlInAs layers in the waveguide structure aredoped to be semi-insulating, can block electric current up to highvoltage bias (>20 V). This can further simplify the device manufacturingprocess because no additional isolation may be needed, such that theisolation regions shown in FIGS. 4A and 4B are optional or omitted.

The currently most preferred embodiment of the present disclosure is alow optical loss passive waveguide core structure which can be used inMid-IR opto-electronic devices, especially in combination with QCLactive materials. This is particularly useful in the case of laser witha relatively thick active region emitting at long wavelengths, such as aQCL emitting in the Mid-IR range and beyond.

For some devices, a waveguide core section (or sections) butt-jointedwith an active (=light-emitting) core section (or sections) isdesirable. The waveguide core material is chosen so that the opticalmode propagates with as little loss as possible at the butt-joint. Thisis partly a problem of growth; in addition, though, the material of thewaveguide core is desirably chosen for an appropriate refractive index,usually identical to the index of the active core. If the waveguide coreis undoped or low-doped, propagation loss through the waveguide willinclude no or very little free-carrier absorption. As noted, in someembodiments, it would also be very advantageous if the waveguide corewere not simply undoped but were semi-insulating so that currentinjected into the active core not leak into the waveguide and not bewasted.

Therefore, a desirable low loss waveguide structure has a core made ofalternating undoped (or Iron doped) GaInAs/AlInAs layers, as shown inFIG. 4C. The core is sandwiched between top and bottom InP claddinglayers (n⁻ doped), like the QCL core. The total thickness of the passivewaveguide core is equal to that of the QC laser core. The thickness of apair of GaInAs/AlInAs should be larger than 0.1 μm. The ratio of thethicknesses of GaInAs and AlInAs is designed in such a way that theeffective index of the optical mode in the passive waveguide equals thatin the waveguide with QC laser core; the ratio of thicknesses also willbe larger than 1% and smaller than 99% (not one pure material)—this factis due to the range of refractive indices aimed for. The size of theoptical mode in the passive waveguide should be similar to that in thewaveguide with QC laser core. FIGS. 5A and 5B show the simulated opticalmode of two passive waveguides with different thickness ratios. FIG. 5Ashows the optical mode in the passive waveguide structure with aGaInAs/AlInAs thickness ratio of 50/50. Here, a pair of GaInAs andAlInAs layers has a thickness of 0.5 μm. The effective index is 3.169,which is slightly lower than the target value (index of the mode in aparticular QCL active section) of 3.2172. FIG. 5B shows the optical modeaccording to simulation in the passive waveguide structure with aGaInAs/AlInAs ratio of 68/32. The effective index is 3.207, whichmatches the particular active core's effective index reasonably well.

The low-loss waveguide embodiment with doping can block electric currentwith high voltage bias (>20 V), thanks to its containing thick enoughAlInAs layers, which are grown doped to be semi-insulating. FIG. 6 showsa test voltage-current (VI) curve of a square mesa of a passivewaveguide structure with such doping. It shows no obvious leakagecurrent up to a voltage bias higher than 25 V.

A DBR QCL wafer with this passive waveguide structure was made. On oneindividual wafer, both a regular DBR QCL (a QCL having the same activewaveguide in the gain and the DBR sections) and DBR QCL with passivewaveguide (using the passive waveguide core to replace the QCL core inthe front and back DBR sections) were fabricated. FIG. 7 shows thelight-current-voltage (LIV) curves of a DBR QCL with passive waveguideand those of a regular DBR QCL from the same wafer and with the samestripe width. The LIVs are similar. Since this is the first such wafergrown, the fabrication (especially at the transition area [the buttjoint] between the gain and DBR sections) is not perfect. The data shownhere are thus only preliminary results. High output power and possiblewider tuning range are expected in the future with the DBR QCL with thepassive waveguide. But what is seen is that lasing can be achieved atlower voltages, even in this first attempt.

Embodiments herein are desirably used in a pulsed mode, butcontinuous-wave mode may be useful in some applications. Laser pulseduration may be from about 1 ns to about 1 ms. In some embodiments, thepulse width at FWHM is about 1 ns, 2 ns, 3 ns, 4 ns, 5 ns, 6 ns, 7 ns, 8ns, 9 ns, 10 ns, 20 ns, 50 ns, 60 ns, 70 ns, 80 ns, 90 ns, 100 ns, 200ns, 300 ns, 400 ns, 500 ns, 600 ns, 700 ns, 800 ns, 900 ns, 1 μs, 10 μs,100 μs, or 1 ms. In some embodiments, devices embodied herein may bedesigned to fire all laser sections simultaneously, individually, and/orin a sequential or programmed order.

Embodiments may be used in any number of methods wherein IR radiation,and particular IR laser radiation would be advantageous. Particularapplications include IR absorbance or reflectance measurements, IR andFTIR spectroscopies, Raman spectroscopy, gas and/or chemical weaponsdetection, chemical dynamics and kinetics measurements, thermalexperiments, etc. In one embodiment, the embodiments are used in IRabsorbance measurements to identify molecular compositions.

While the present invention has been described at some length and withsome particularity with respect to the several described embodiments, itis not intended that it should be limited to any such particulars orembodiments or any particular embodiment, but it is to be construed withreferences to the appended claims so as to provide the broadest possibleinterpretation of such claims in view of the prior art and, therefore,to effectively encompass the intended scope of the invention.Furthermore, the foregoing describes the invention in terms ofembodiments foreseen by the inventor for which an enabling descriptionwas available, notwithstanding that insubstantial modifications of theinvention, not presently foreseen, may nonetheless represent equivalentsthereto.

1. A semiconductor optical emitter having an optical mode and a gainsection, the emitter comprising a low loss waveguide structure made ofalternating layers of at least two semiconductor materials A and B,having refractive indexes of N_(a) and N_(b), respectively, with aneffective index N_(o) of the optical mode in the low loss waveguidebetween N_(a) and N_(b), wherein a ratio of thicknesses of the materialsA and B is selected in order to form the low loss waveguide with theeffective index N_(o) that is within a desired error margin of identicalto a refractive index of the gain section.
 2. The emitter of claim 1wherein at least one of the semiconductor materials A and B has asufficiently large band gap that the passive waveguide structure blockscurrent under a voltage bias of 15 V.
 3. The emitter of claim 1 whereinat least one of the semiconductor materials A and B has a sufficientlylarge band gap that the passive waveguide structure blocks current undera voltage bias of 20 V.
 4. The emitter of claim 1 wherein at least oneof the semiconductor materials A and B has a sufficiently large band gapthat the passive waveguide structure blocks current under a voltage biasof 25 V.
 5. The emitter of claim 1 wherein material A is AlInAs andmaterial B is GaInAs.
 6. The emitter of claim 5 wherein the AlInAs andthe GaInAs are left undoped.
 7. The emitter of claim 5 wherein theGaInAs is left undoped and the AlInAs is doped with a deep trap elementor elements.
 8. The emitter of claim 7 wherein the deep trap element orelements is one, or a combination, of iron and titanium.
 9. The emitterof claim 7 wherein the deep trap element is iron.
 10. The emitter ofclaim 7 wherein the deep trap element is Ruthenium.
 11. The emitter ofclaim 1 wherein No is within a 5% error margin of identical to arefractive index of the gain section.
 12. The emitter of claim 1 whereinthe gain section is butt-jointed with the low loss waveguide.
 13. Theemitter of claim 1 wherein the size and shape of the optical mode(s) inthe low loss waveguide and gain section are within a 10% error margin ofequal.